We present a novel, high efficiency system to modulate
the amplitude of the RF output signal of a current mode class-D on-board
amplifier (CMCD). The amplitude-modulating wave is used as the reference signal
of a Pulse Width Modulated (PWM) controller that switches the gate of high
frequency MOSFETs in a buck converter configuration. Current feedback is used
to directly control the current in the transmit coil. We successfully showed
the system can handle up to 20kHz AM signal and higher, and load insensitivity
was significantly improved comparing with that of a tuned voltage-mode device.